A printing device often receives an image specification in the form of objects or object descriptions. For example, an image might be specified in terms of graphical shapes such as lines, boxes, circles, characters, etc.. These objects or object descriptions are received sequentially, and might overlay each other on the printed page.
Most printing devices are raster-type devices, meaning that they apply a raster image to a printed medium. A raster image comprises a two-dimensional array of picture elements or “pixels.” When printing, a printing device converts received graphics objects into raster images. The printing device generates the raster image by sequentially rendering received objects onto a high resolution pixel map (commonly termed a “bit map”). Each pixel rendered onto the high resolution pixel map is represented by several bytes to store such information as intensity, grey scale, color plane, and the like.
In sequentially rendering each object onto the high resolution pixel map, the printing device may perform operations that depend upon whether any pixels affected by the in-process object have already been rendered onto the pixel map during the rendering of a previous object. Such operations are commonly referred to as destination dependent operations. These operations may include, for example, color blending, foreground/background processing, and transparency processing.